Journal Article

Particle energy cascade in the intergalactic medium

M. Valdés, C. Evoli and A. Ferrara

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 404, issue 3, pages 1569-1582
Published in print May 2010 | ISSN: 0035-8711
Published online May 2010 | e-ISSN: 1365-2966 | DOI:
Particle energy cascade in the intergalactic medium

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We study the development of high-energy (Ein≤ 1 TeV) cascades produced by a primary electron of energy Ein injected into the intergalactic medium (IGM). To this aim we have developed the new code medea (Monte Carlo Energy Deposition Analysis) which includes Bremsstrahlung and inverse Compton (IC) processes, along with H/He collisional ionizations and excitations, and electron–electron collisions. The cascade energy partition into heating, excitations and ionizations depends primarily not only on the IGM ionized fraction, xe, but also on redshift, z, due to IC on cosmic microwave background (CMB) photons. While Bremsstrahlung is unimportant under most conditions, IC becomes largely dominant at energies Ein≥ 1 MeV. The main effect of IC at injection energies Ein≤ 100 MeV is a significant boost of the fraction of energy converted into low-energy photons (hν < 10.2 eV) which do not further interact with the IGM. For energies Ein≥ 1 GeV CMB photons are preferentially upscattered within the X-ray spectrum (hν > 104 eV) and can free stream to the observer. Complete tables of the fractional energy depositions as a function of redshift, Ein and ionized fraction are given. Our results can be used in many astrophysical contexts, with an obvious application related to the study of decaying/annihilating dark matter (DM) candidates in the high-z Universe.

Keywords: intergalactic medium; cosmology: theory; dark matter; diffuse radiation

Journal Article.  6355 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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